Flow control

Ice deterioration

Structural measures

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pump a slurry of water and riverbed sediments

technique to fracture an ice cover and cause pre-

onto the ice, at least for rivers with silt or sand

mature breakup (Ferrick and Mulherin 1989). The

bed material. Moor and Watson (1971) described

rapid release of a pulse of water is intended to

tests in which dusting coverage similar to aerial

cause the ice cover to fracture and run with a

dusting was obtained by pumping. While the cost

smaller increase in stage than required for most

of treating a 100-ft-wide strip was less than for

natural runoff events. This method could be help-

aerial application (

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.09 per lineal foot in 1970

ful in the case of a tributary stream that jams as it

dollars), a disadvantage is the time required to

encounters an intact ice cover at its confluence

cover large areas, as required in most ice jam pre-

with a larger river. With adequate flow control on

vention applications.

the larger river, it might be possible to cause the

ice on the larger river to break up prior to the

Flow control

smaller river, providing room for the smaller riv-

The control of river discharge has been used

er's ice run to enter. The dam release must be

both to increase ice cover stability and prevent

quite rapid and near to the problem site so that a

jamming and to intentionally cause river ice

sharply defined wave passes through the ice. Al-

breakup. Flow control is often used during the

though clearing the ice cover on the Missouri Riv-

fall freezeup period to allow a thinner, more sta-

er prior to breakup on the Yellowstone River

ble ice cover to form in a reach where natural flow

should reduce the threat of ice jam flooding, Fort

velocities would cause ice floes to underturn or

Peck dam is much too far away to provide such a

shove and thicken into a jam. This form of flow

pulse of water.

control may also be used to improve the perfor-

mance of ice control structures by providing

Icebreaking

reduced, steady flows during ice cover formation

Icebreaking is another nonstructural alterna-

(Deck 1984). Similarly, flow control during the

tive for clearing a channel or weakening its ice

spring could be used to prevent ice cover break-

cover in advance of a natural breakup. As de-

up, delay breakup until significant deterioration

scribed in the flow control section above, ice-

of the ice cover has occurred, or reduce peaks in

breaking on the Missouri River in advance of the

water discharge and resultant flooding. This

breakup of the Yellowstone River ice cover would

approach, however, requires a dam with suffi-

help mitigate ice-related flooding. The use of ice-

cient storage capacity located near enough to the

breaking ships is highly effective but requires

flooding problem area.

suitable vessels on-site, which is not the case in

In the case of the BufordTrenton area, flow

the BufordTrenton area. In rivers that are isolat-

control is employed by Fort Peck Dam on the

ed from navigable waterways or too shallow for

Missouri to reduce flood peaks during the break-

navigation by icebreaking ships or tugs, air-cush-

up of the Yellowstone River. This also increases

ion vehicles (Kankakee and St. Lawrence Rivers)

the tendency of the Missouri River to break up

or amphibious landing craft (Buffalo River) have

several weeks after the Yellowstone River and

also been used.

limits the supply of ice feeding any ice jams that

might occur within the Irrigation District during

Early warning

the breakup period on the Yellowstone River.

While not a flood mitigation measure, an early

During the 1986 event, Fort Peck releases were

warning system can be quite useful in minimiz-

reduced from 12,000 cfs on 1 March to 1,000 cfs on

ing flood losses. The ice jam potential rating

9 March to lessen flooding (USACE 1986).

scheme presented earlier can be used to provide

Unfortunately Fort Peck is far upstream from the

some indication of whether ice-related flooding is

BufordTrenton area and can control only a small

likely. In addition, there are numerous water level

portion of the flow causing flooding (the drain-

gages along the Missouri and Yellowstone Rivers

age area above Fort Peck Dam is 57,500 mi2 vs.

in the vicinity of the Irrigation District that are ei-

164,500 mi2 above Williston). Further, close moni-

ther no longer in use or not active during the win-

toring of conditions on the Yellowstone is a must,

ter months. Reactivating gages in the District and

since the travel time of water from Fort Peck Dam

for a good distance upstream, and adding tele-

to the District (about 200 river miles) is on the or-

phone, radio or satellite relays, could provide an

der of four days.

early warning system for increased flows, ice

Flow control, in the form of a rapid increase of

movement or increasing stages. Direct ice motion

discharge from a dam, has also been tested as a

sensors could also be added to the gaging stations

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